The materials which are used for making rolls for rolling mills, dies, punches, etc. are required to having high degrees of toughness, impact resistance, high-temperature strength, etc. Therefore, it has been usual to employ cast steels, tool steels, etc. for those purposes. These materials have, however, the drawback of being low in wear resistance and being, therefore, capable of making only parts having a short life.
Attempts have, therefore, been made to improve the drawback of those materials by employing, for example, an ultrahard WC--Co alloy composed of a hard phase of WC and a binding phase of Co, or a cermet alloy composed of a hard phase of titanium compound, such as TiCN, and a binding phase of Ni.
The ultrahard WC-Co alloy consists mainly of WC and contains 15 to 25% by weight of Co which binds WC. As WC is essentially of high wear resistance, the alloy can make e.g. rolls of high wear resistance, as compared with the rolls formed from any conventional materials, such as cast or tool steels.
The ultrahard WC--Co alloy has, however, a number of drawbacks including (1) resulting in a part having a large weight, as WC, of which it mainly consists, has a high specific gravity (about 15), (2) being difficult to employ for making any part to be exposed to heat, as WC is liable to oxidation, particularly at high temperatures, and (3) being low in breaking or chipping resistance, as it contains only a small amount of binding phase and is substantially a lump of carbide particles.
The attempt to improve the chipping resistance of the ultrahard WC--Co alloy by increasing the amount of its binding phase (i.e. the amount of Co which it contains) results not only in a sharp reduction of wear resistance, but also in a reduction of hardness and toughness, if its Co extent exceeds 25% by weight, as is well known in the art. Therefore, a practically acceptable ultrahard WC--Co alloy is one containing 15 to 25% by weight of a binding phase.
The first cermet alloy appeared in the market in 1971. A great many attempts have since been made to obtain improved cermet alloys by employing carbides of Group IVa, Va and VIa metals, such as WC, TaC and NbC, for replacing a part of a titanium compound such as TiCN. The cermet alloys play an important role in the manufacture of cutting tools.
It is, however, known that if a cermet alloy contains over 40% by weight of a metal binding phase consisting basically of Ni, the binding phase between hard particles has an average thickness (m.f.p.) exceeding an adequate range for the alloy and resulting therefore in such a great reduction in strength of the alloy as to make it unsuitable for practical use, and that the amount of the binding phase in any such alloy should, therefore, be limited to a maximum of 40% by weight (see e.g. Hisashi Suzuki: "Ultrahard Alloys and Sintered Hard Materials" (Published by Maruzen), pages 307 to 372).
The conventional cermet alloys are, therefore, low in toughness and impact strength and unsatisfactory in breaking resistance, too, though they are light in weight and high in oxidation resistance, as compared with the ultrahard WC--Co alloys. Therefore, they have found only a limited scope of application to hot and cold working rolls, extrusion dies, or wire drawing dies.
It is usual to add a component, such as Mo.sub.2 C or WC, to a cermet alloy to improve the wetting property of its hard phase composed of particles of a titanium compound (e.g. TiCN) with its binding metal phase. The component undergoes dissolution in the binding metal phase and precipitation in the hard phase during a sintering process and forms a structure surrounding the particles of the titanium compound and thereby improving their wetting property with the binding metal phase. Therefore, the conventional cermets usually comprise a composite carbonitride having a cored structure formed by a central portion which is rich in Ti, and a peripheral portion which is rich in e.g. WC or Mo.sub.2 C, but lean in Ti (see e.g. Japanese Patent Publication No. 51201/1981, or Japanese Patent Application laid open under No. 73857/1986, 210150/1986 or 201750/1986).
When any such cermet is used to make, for example, a roll, the benefit of the titanium compound which it contains cannot be obtained from the roll, since the surface of the carbonitride which is exposed as a result of the wear of the binding metal phase is easily oxidizable and soft, as it is lean in Ti, while it is rish in W. As the component, such as WC or Mo.sub.2 C, forms the peripheral portion, the particles of the carbontrile grow until they contact one another. The contacting portions of the particles are likely to become a source of fine cracks and open a path for the propagation of cracks. The more contacting portions the particles have, the lower the fracture toughness of the alloy becomes. The presence of those contacting portions also lowers the breaking resistance of the alloy. However, if the amount of the component, such as WC or Mo.sub.2 C, which is added is reduced to decrease those contacting portions, the high-temperature strength of the alloy is greatly lowered. Therefore, the addition of some of any such component is essential and the presence to some extent or other of such contacting portions is unavoidable.
Attempts have also been made to manufacture rolls and other parts having improved properties by employing a composite of different materials instead of a single material.
Practical use has come to be made of, for example, finishing rolls which are manufactured from an ultrahard WC--Co alloy used as a single material because of its high wear resistance. This alloy has, however, a high specific gravity (about 15), as already stated. Its specific gravity is nearly twice as high as that of cast or tool steel. The rolls manufactured from it are, therefore, greater in weight, and more likely to chatter or vibrate when they are operating. The greater weights of the rolls produces a greater force of inertia resulting in a greater difference between the peripheral velocity of the rolls and the speed at which the material to be rolled is passed therebetween. This difference creates between the rolls and the material to be rolled a large amount of slip which exerts an adverse effect of the quality of the material to be rolled.
A composite roll which is lighter in weight has been proposed to overcome the drawbacks of the roll made of a single material and comprise, for example, a core (inner layer) formed from a lightweight TiC--Ni cermet having a specific gravity of 5.1 and an outer layer formed from a wear-resistant ultrahard WC--Co alloy (see Japanese Patent Application laid open under No. 56147/1978). This roll, however, is easily broken and lacks reliability, since the cermet forming its inner layer is inferior in toughness to the ultrahard alloy. The residual stress which is due to the difference in coefficient of thermal expansion between the inner and outer layers of the roll is likely to disable them to maintain a proper bond and the roll is, therefore, likely to break, particularly when it is used for hot working.
There is also known a roll made by fitting a pressed cylinder of a WC--Co alloy about a sintered WC--Co alloy column and sintering them together (see Japanese Patent Application laid open under No. 84711/1976). This roll, however, still leaves unsolved the problem which is due to the high specific gravity of the ultrahard WC--Co alloy, though it may no longer have any problem due to the difference in coefficient of thermal expansion between the inner and outer layers.
There is also known a composite heat-resistant alloy part comprising a core formed from a heat-resistant alloy and having an outer surface coated by HIP with a powder of a corrosion-resistant alloy, such as a Ni-, Co- or or Fe-based alloy having a higher chromium content than the core alloy (see Japanese Patent Application laid open under No. 62103/1980). This part is, however, not fully satisfactory in wear resistance, despite its improved strength and corrosion resistance at elevated temperatures.
Under these circumstances, it is an object of this invention to provide a cermet alloy of high toughness, impact resistance and hardness which can be used for a wide scope of application including the manufacture of rolls and dies.
It is another object of this invention to provide a composite mechanical part of improved breaking resistance which is made by employing the cermet alloy of this invention and another appropriate material.